encoding/pem: eliminate allocations for newlines during encoding

benchmark           old MB/s     new MB/s     speedup
BenchmarkEncode     243.20       279.89       1.15x

benchmark           old allocs     new allocs     delta
BenchmarkEncode     1370           4              -99.71%

Change-Id: I3920bcc04b6dd89efa5da89db5594d4434426d74
Reviewed-on: https://go-review.googlesource.com/1924Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/pem/pem.go

@@ -171,6 +171,8 @@ type lineBreaker struct {
 	out  io.Writer
 }
 
+var nl = []byte{'\n'}
+
 func (l *lineBreaker) Write(b []byte) (n int, err error) {
 	if l.used+len(b) < pemLineLength {
 		copy(l.line[l.used:], b)
@@ -190,7 +192,7 @@ func (l *lineBreaker) Write(b []byte) (n int, err error) {
 		return
 	}
 
-	n, err = l.out.Write([]byte{'\n'})
+	n, err = l.out.Write(nl)
 	if err != nil {
 		return
 	}
@@ -204,7 +206,7 @@ func (l *lineBreaker) Close() (err error) {
 		if err != nil {
 			return
 		}
-		_, err = l.out.Write([]byte{'\n'})
+		_, err = l.out.Write(nl)
 	}
 
 	return
@@ -248,7 +250,7 @@ func Encode(out io.Writer, b *Block) error {
 				return err
 			}
 		}
-		if _, err := out.Write([]byte{'\n'}); err != nil {
+		if _, err := out.Write(nl); err != nil {
 			return err
 		}
 	}

src/encoding/pem/pem_test.go

@@ -6,6 +6,7 @@ package pem
 
 import (
 	"bytes"
+	"io/ioutil"
 	"reflect"
 	"testing"
 )
@@ -116,6 +117,24 @@ func TestLineBreaker(t *testing.T) {
 	}
 }
 
+func BenchmarkEncode(b *testing.B) {
+	data := &Block{Bytes: make([]byte, 65536)}
+	b.SetBytes(int64(len(data.Bytes)))
+	for i := 0; i < b.N; i++ {
+		Encode(ioutil.Discard, data)
+	}
+}
+
+func BenchmarkDecode(b *testing.B) {
+	block := &Block{Bytes: make([]byte, 65536)}
+	data := EncodeToMemory(block)
+	b.SetBytes(int64(len(data)))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		Decode(data)
+	}
+}
+
 var pemData = `verify return:0
 -----BEGIN CERTIFICATE-----
 sdlfkjskldfj